How to solve the bubbles produced by the paint?
With the rapid development of the coating industry, the variety of coatings, grade upgrade, people put forward higher requirements for the protection and decoration of the coating, therefore, the surface state of the coating is of great concern. The existence of air bubbles seriously affects the appearance of the coating film, often causing shrinkage, pinholes, blemishes, fish eyes and other ills. Therefore, the problem of coating bubbles has not only affected the protective effect of the coating film, but also greatly affected the decorative effect of the coating film.
Characteristics and classification of air bubbles
In the process of coating production, bubbles appear as disturbing factors, so that the coating products in the application of surface defects, both detrimental to the appearance, but also affect the corrosion resistance and weatherability of the coating film.
Bubbles have size and can contain liquid, vapor, other gases or crystalline matter. The size of the bubble depends on the adhesion of the coating to the surface of the substrate, the pressure of the liquid or gas inside the bubble, and the extent to which the coating film is stretched and reversed in order to maintain a balance with the adhesion of the coating substrate. According to its form, bubble can be divided into two kinds: bubble and foam. Bubble is a single spherical tiny air dispersed in a highly viscous medium. Most of the bubbles produced by solvent-based coatings are of this type. Foam, on the other hand, appears in media that are prone to foaming. When this kind of medium is dispersed, due to the large filling density of air, a large number of bubbles are often produced, because the density difference between air and liquid is very large, the bubbles will quickly rise to the liquid surface, forming a small amount of liquid to form a dense collective of bubbles separated by a liquid film. Water-based paint bubbling is mostly of this kind.
What are the causes of bubbling?
Blistering usually occurs along with the aging process of coating, in this sense, blistering can be divided into two kinds of expansion blistering and corrosion blistering. There are several reasons for the blistering of the coating film.
PART1. Improper surface treatment of the substrate
Substrate surface oxides, soluble salts and other substances are the triggering point of coating blistering. As the substrate surface is stained with wax, oil, dust, soluble salt and other substances, or the substrate surface has moisture, water droplets, volatile liquid presence, or surface treatment, the workpiece concave rust, moisture, etc., resulting in coating film drying, the coating in these places and the substrate surface adhesion is not firm, moisture accumulation in the low adhesion area, so that the accumulation of liquid or gas phase in these areas. When the temperature changes, the vapor pressure generated by its thermal expansion is the main reason for bubble generation. If in a hot and humid environment, the dual action of vapor pressure and osmotic pressure will intensify bubble generation. The rust, oxidation and other substances in the coating or from the coating film microscopic channels within the water, gas, corrosion medium reaction, the generation of gas, coupled with the influence of ambient temperature, humidity, will also cause the coating blistering.
PART2. The gas within the coating is released with the increase of temperature
In the wet film formed by coating, there are still about 10% of volatile solvents, other about 90% of solvents have been volatilized to the atmosphere during the painting process, and some other improper volatilization of solvents in the coating will also cause residue and form bubbles. The retention ability of solvents is related to the evaporation rate of solvents and external conditions, but also to their solvent interaction and the adsorption of fillers, etc. The residual solvent in the coating is like a plasticizer, which softens the coating film. Since solvents are sensitive to water, residual solvents can increase the absorption of water in the coating film and the transfer of moisture in the coating. If the adhesion of the coating itself is not firm, the decrease of its water resistance will cause the loss of adhesion, resulting in blistering of the coating.
PART3. Gas residue in porous substrate
Solvent can also cause the coating film to blister on porous substrates (such as cement, wood). Usually organic coatings are applied on inorganic substrates, and blistering is easy to occur when the surface temperature is high. At this time, the bubble generation speed is very fast, easy to break and reveal the substrate. This kind of bubble can be eliminated by applying penetrating primer. Penetrating primer is usually low viscosity, thin film, can dry quickly, and have enough time to penetrate. Avoiding high temperature construction time can also avoid such bubbles.
PART4. Absorption of gas or liquid on the surface of pigment or filler
Soluble filler in paint is the cause of many bubble problems. Soluble pigment absorbs moisture through the coating and becomes a solution. Due to osmosis, water will be pulled through the coating to the concentrated solution. Also if the soluble pigment stains the substrate or any surface between the coatings, the same phenomenon will occur. Therefore, blistering is inevitable as long as the coating contains soluble substances.
Once you understand the causes of bulging and pinhole formation, you have to find the right solution.
From strengthening construction management and paint correct brushing method two aspects to solve
1, construction to avoid high temperature, humidity and other bad weather, substrate surface humidity to maintain at 3 ℃ or more, relative humidity is not higher than 85%.
2、Treat the substrate thoroughly, keep the surface clean, no residual electrolyte, dust, oil and other impurities that affect the adhesion of the paint.
3, (emphasis) when brushing the paint, use thin coating with multiple passes, increase the brushing time, so that the solvent between layers is fully evaporated and reduce the amount of residue.
4、Crossing method of brushing to ensure uniform paint film thickness.